The protozoan Tetrahymena will be used as a model organism to study the effect of incorporation of a sterol different from the cell's native compound, on enzyme activities of the endoplasmic reticulum, Specifically, the normal membrane component, tetrahymanol, will be replaced by ergosterol (the native sterol of yeast). The normal and the sterol-replaced cells will be compared with respect to the activity and characteristics of the enzyme system which produces linoleic acid in this organism. This enzyme system thus far appears to require the coenzyme A derivative of the substrate, 14C-oleic acid, and is at present referred to as the oleoyl CoA desaturase. Various kinetic properties and the susceptibility of the enzyme system to changes in the endoplasmic reticulum lipid composition will be studied. In addition, the enzyme will be purified, and the quantity of enzyme present in both normal and sterol-replaced cells will be determined. The decrease in formation of linoleate which has been observed when living cells are treated with ergosterol will be studied in vitro to determine whether this decrease is due to quantity of oleoyl CoA desaturase or to activity of this or other enzymes.